1. Field of the Invention
The present disclosure relates to a compound semiconductor device in which a compound semiconductor layer epitaxially grown on a silicon substrate is formed via a buffer layer made of aluminum nitride, as well as to a method for producing the same, and a resin-sealed type semiconductor device.
2. Description of the Related Art
In recent years, as a material of a semiconductor device, development of a nitride-based semiconductor material, which is a wide-bandgap semiconductor, is actively carried out. As a characteristic feature of the wide-bandgap semiconductor, it can be mentioned that the wide-bandgap semiconductor has an insulation breakdown voltage larger than that of silicon (Si), which is a general semiconductor, by an order of magnitude.
With conventional Si, a drift layer in which electrons travel needs to be made long in order to obtain a power semiconductor device having a high breakdown voltage. In contrast, gallium nitride (GaN) provides an equivalent breakdown voltage with a short drift layer (about 1/10  of Si). In this case, when a situation of allowing an electric current to flow in the semiconductor device is considered, the drift layer becomes a resistance layer, so that the on-resistance of the semiconductor device becomes smaller when the drift layer is shorter. Theoretically, assuming that the mobility and the dielectric constant of a semiconductor are of the same degree, the on-resistance of the semiconductor device exhibiting a certain predetermined breakdown voltage is inversely proportional to the cube of the insulation breakdown electric field that the semiconductor material has. In other words, with the same chip area, an on-resistance lower by about 1/1000  can be achieved in a GaN device as compared with a Si device.
A nitride-based semiconductor material can form various mixed crystals with GaN, aluminum nitride (AlN), and indium nitride (InN), so that the nitride semiconductor material can make a heterojunction as with a conventional arsenic semiconductor material such as gallium arsenic (GaAs). In particular, the heterojunction of the nitride-based semiconductor has a characteristic feature such that high concentration of carriers are generated at the interface by spontaneous polarization or piezo-polarization even in a state in which doping of impurities is absent. As a result, in a lateral-type device in which an electric current is allowed to flow in a direction parallel to the silicon substrate using the heterojunction of GaN/AlGaN, a device for large electric power having a low on-resistance with a large electric current can be achieved.
Further, the nitride-based semiconductor material can be epitaxially grown on a silicon substrate via a buffer layer made of aluminum nitride. In other words, though it is necessary to use an expensive silicon carbide (SiC) substrate with the same wide-bandgap semiconductor material in the case of a SiC device, it is possible to use a silicon substrate in the case of a nitride-based semiconductor device, so that reduction of costs and increase in the diameter can be achieved.
In the meantime, the nitride-based semiconductor device in which the nitride-based semiconductor layer has been formed on the silicon substrate (wafer) is divided into semiconductor devices by performing dicing along a scribe lane as with a conventional silicon device or GaAs device. In this dicing step, after the wafer is bonded to a dicing tape, the wafer is subjected to a cutting process along the scribe lane while a thin-type grindstone having a disk shape, which is known as a dicing blade, is rotated at a high speed.
In this dicing step, fragmentation, cracks, and crystal defects of the semiconductor layer, which are called chipping, are generated in the scribe lane if a blade kind, a rotation number, a dicing speed, and the like are not appropriately selected. Further, when the chipping or crystal defects generated in the scribe lane reach an element formation region within the semiconductor device, deficiency of electric characteristics or deficiency of reliability caused by penetration of moisture is generated.
Generally, the blade kind, the rotation number, and the dicing speed are appropriately selected in order to eliminate deficiency generation in the semiconductor device caused by chipping or crystal defects. A scribe lane width is set so that the deficiency may remain within the scribe lane even when chipping or crystal defects are generated or when moisture penetrates via the buffer layer.
In the silicon device, a structure of suppressing chipping in the semiconductor device is known. For example, PTL 1 discloses a structure in which a film is formed on a scribe lane between a plurality of semiconductor elements formed on a semiconductor wafer. According to this structure, progression of a stress that generates the chipping can be absorbed or alleviated with a wall of this film, so that suppression of the chipping can be expected.
On the other hand, in a nitride-based compound semiconductor device, a structure in which an aluminum nitride layer is formed as a surface protection film is known. For example, PTL 2 discloses a structure in which an AlN layer is formed as a surface protection film on an upper-side surface of an AlGaN layer. According to this production method, the upper-side surface of the AlGaN layer is covered with the AlN layer before cracks are generated, so that it can be expected that the surface will be a flat surface without cracks.